Originally hydrocarbon cracking was accomplished pyrolytically by using Dubbs thermal cracking units similar to visbreakers. When naturally occurring acidic silica-alumina clays were found to be effective cracking catalysts, fluidbed processing was adopted to improve cracking selectivity for less gas and more gasoline and distillate production relative to thermal cracking. These fluid bed crackers exhibited long residence time cracking in the range of 10-30 seconds with substantial product backmixing in the fluid bed reactor. Because there was no means of disengaging the catalyst from the hydrocarbon vapors, the entire reactor vessel served as reaction volume, not just the dense fluid bed. The hydrocarbon product and cracking catalyst were entrained together to the reactor cyclones thereby substantially increasing the overall cracking residence time.
When new high active zeolite catalyst were developed, the residence time had to be drastically reduced to avoid overcracking by the very active catalyst. Bed crackers were converted to riser transport reactors with a small bed at the end of the riser. As catalysts improved, the dense bed component of the reactor was eliminated resulting in all riser cracking with a residence time of about 1-5 seconds. Riser cracking resulted in much more selective cracking products (less gas, more gasoline) due to a reduction in backmixing and product degradation via overcracking in the riser.
Along with the new catalysts and new cracking schemes to accommodate these catalysts, a need was created to rapidly separate the highly reactive cracking catalysts from the cracked hydrocarbon vapor to preserve the selection riser cracking products and avoid further reactions to undesirable byproducts after exiting the reactor riser. In addition to rapidly separating the cracking catalysts and cracked hydrocarbon product, a means to collect and recycle the cracking catalysts was needed to insure the most efficient use of the overall catalytic cracking scheme. With the more highly active catalysts, there was more concern over post riser cracking in the dilute phase (catalyst entrained in the gas leaving the riser and proceeding to the reactor vessel cyclones). Initially only catalytic aftercracking was contemplated. Various means were developed for separating the catalyst particles from the cracked product gas. The most common and now traditional means for separating the catalyst and product gases from a riser reactor are the cyclone separators. Other catalyst riser reactor separation methods exist which rely on either inertial forces or centrifugal forces.
For example, apparatus has been developed to roughly separate cracking catalysts and vapor products by using a pipe tee arrangement to change the direction of the catalyst-gas mixture issuing from the riser reactor by 180.degree. and thereby obtain rough separation based on inertial forces.
Other variations of inertial separation use a horizontal deflection plate at the end of the riser reactor instead of a pipe tee to change the direction of the feed mixture by 180.degree. and thereby achieve separation. Illustrative methods and apparatus are taught by Van Dommelen, U.S. Pat. No. 2,947,577, Strickland et al., U.S. Pat. No. 3,957,443, and Pfeiffer et al., U.S. Pat. No. 4,756,886.
Hutchings, U.S. Pat. No. 3,247,651 describes an inertial type separator that comprises an elbow at the end of the riser reactor to change the direction of the feed mixture by 90.degree. instead of 180.degree.. This change in direction results in a poor separation with approximately 60% of the cracking catalyst falling back into the riser reactor.
Effectively, the inertia type separators allow the vapor product to remain in contact with a portion of the cracking catalyst after separation thereby resulting in unwanted side reactions and byproducts.
Prior art separators such as Jewell, U.S. Pat. No. 2,439,811 teach a separation technique, based on centrifugal force rather than inertia type separation. In essence, the centrifugal separators use a semicircular deflection means at the top of a riser reactor to change the direction of the feed mixture by 180.degree. and to impart a centrifugal force on the feed mixture causing separation of the vapor product and cracking catalyst. The separated catalyst is then deposited in the bottom of the separator vessel and the vapor product is drawn from the separation vessel.
A different type of centrifugal separator is taught by Ross et al., U.S. Pat. No. 2,878,891 and McKinney et al., U.S. Pat. No. 4,061,502. These separators employ a curved riser reactor pipe to create the centrifugal force. The curved riser reactor causes the cracking catalysts to travel against the outside wall of the separator while the vapor product remains closer to the inside wall where it can be removed.
Another type of centrifugal separator is taught by Evans, U.S. Pat. Nos. 2,901,420 and 2,888,096. The separators disclosed therein are essentially horizontal centrifugal separators or horizontal cyclone separators where the feed mixture is tangentially fed into the horizontally mounted cylinder. The mixture travels around the walls of the cylinder where centrifugal forces act upon it causing the cracking catalysts and vapors to separate. The vapors are removed axially through the centrally disposed pipe in the cyclone cylinder, while the cracking catalysts are removed through an opening in the bottom of the cylinder.
Although the centrifugal separators which have been in use for some time achieve efficient separation; at times in the area of 90-99% efficiency, separation is not rapid. Therefore the vapor product remains in contact with a significant portion of the cracking catalysts during the separation process thereby producing unwanted side reactions and byproducts.
Also the collection efficiencies of these separators degrade significantly when process conditions vary or upsets occur.
In addition to the vapor product remaining in contact with the catalyst during separation causing unwanted side reactions and byproducts, further side reactions occur and byproducts result by thermal cracking of the separated vapor product due to uncontrolled post riser reactor residence time. Post riser reactor residence time is defined as the time the cracked hydrocarbon product remains in the separator, after exiting the riser reactor. The normal cracking temperature of 950.degree.-980.degree. F. causes a significant amount of thermal cracking to occur if the post riser reactor residence time is not controlled. This post riser reactor thermal cracking is becoming even more of a problem with the increased demand for high octane gasolines and olefins for alkylation which are produced by a higher than normal riser reactor temperature.